Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst typically induces the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise a porous electrically conductive sheet material, or electrode substrate, upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black, an alloy or a supported metal catalyst such as, for example, platinum on carbon.
A particularly attractive fuel cell is the solid polymer electrolyte fuel cell, which employs a membrane electrode assembly ("MEA"). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
Electrocatalyst can be incorporated at the electrode/electrolyte interfaces in solid polymer fuel cells by applying it in a layer on either an electrode substrate or on the membrane electrolyte itself. In the former case of the, electrocatalyst particles are typically mixed with a liquid to form a slurry or ink which is then applied to the electrode substrate. While the slurry preferably wets the substrate surface to an extent, it is preferred that the slurry not penetrate too deeply into the substrate so that as much of the catalyst as possible will be located at the desired membrane electrolyte interface.
Effective electrocatalyst sites have several desirable characteristics: (1) the sites are accessible to the reactant, (2) the sites are electrically connected to the fuel cell current collectors, and (3) the sites are ionically connected to the fuel cell electrolyte. Electrons and protons are typically generated at the anode electrocatalyst. The electrically conductive anode is connected to an external electric circuit, which conducts an electric current. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode. Electrocatalyst sites are not productively utilized if the protons do not have a means for being ionically transported to the electrolyte. Accordingly, coating the exterior surfaces of the electrocatalyst particles with ionically conductive ionomer coatings has been employed to increase the utilization of electrocatalyst exterior surface area and to increase fuel cell performance by providing improved ion-conducting paths between the electrocatalyst surface sites and the electrolyte. The ionomer can be incorporated in the catalyst ink or can be applied to the substrate after it has been coated with catalyst.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell, thereby resulting in improved performance.
A broad range of reactants can be used in electrochemical fuel cells and such reactants may be delivered in gaseous or liquid streams. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or aqueous methanol in a direct methanol fuel cell (DMFC). The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air.
Solid polymer fuel cells that operate on liquid reactant streams ("liquid feed fuel cells") have somewhat different requirements than those operating on gaseous reactant streams. In particular, the requirements for effectively distributing a liquid reactant stream and making reactant contact with the electrocatalyst layer are different than for a gas stream. For instance, hydrophobic components such as PTFE are typically employed in gaseous feed fuel cells, in part, to render electrodes less wettable and to prevent "flooding". (Flooding generally refers to a situation where the pores in an electrode are so full of liquid, e.g. reaction product water, that the flow of the gaseous reactant through the electrode becomes impeded.) In liquid feed fuel cells, however, it can be desirable to make components in the anode (e.g. catalyst layer) more wettable by the liquid fuel stream in order to improve access of the reactant to the electrocatalyst sites.
In early DMFCs, sulfuric acid was incorporated in the liquid methanol fuel stream in order to enhance proton conduction at the anode. The presence of sulfuric acid however may limit the performance of the fuel cell in other ways and impose constraints on the fuel cell hardware for corrosion reasons. Acid electrolyte additives are no longer considered necessary to obtain reasonable performance from a DMFC. Instead, ionomeric coatings of the anode in the vicinity of the catalyst layer can provide for satisfactory proton conduction. Such an ionomeric coating may also improve wetting and hence access of the aqueous methanol fuel.
While it may seem desirable generally to improve the wetting of a DMFC anode, treatments that improve wetting of the anode per se, do not necessarily result in a net performance improvement. For instance, an ionomer coating also can act as a barrier to the transport of electrons, liquid fuel, and reaction product gases (e.g. carbon dioxide from methanol oxidation) thereby reducing net performance of fuel cells. Thus, the net effect of such treatments is difficult to predict.